1. Field of the Invention
The present invention relates to medical devices that are implanted into the body of an animal, and more particularly to control of electrical power supplied to the implanted medical device from an extracorporeal power supply.
2. Description of the Related Art
A remedy for people with slowed or disrupted natural heart activity is to implant a cardiac pacing device which is a small electronic apparatus that stimulates the heart to beat at regular rates.
Typically a battery powered pacing device is implanted in the patient's chest and has sensor electrodes that detect electrical impulses associated with in the heart contractions. These sensed impulses are analyzed to determine when abnormal cardiac activity occurs, in which event a pulse generator is triggered to produce electrical pulses. Wires carry these pulses to electrodes placed adjacent specific cardiac muscles, which when electrically stimulated contract the heart chambers. It is important that the electrodes be properly located to produce contraction of the heart chambers.
Modern cardiac pacing devices vary the stimulation to adapt the heart rate to the patient's level of activity, thereby mimicking the heart's natural activity. The pulse generator modifies that rate by tracking the activity of the sinus node of the heart or by responding to other sensor signals that indicate body motion or respiration rate.
U.S. Patent Application Publication No. 2005-0096702 describes a cardiac pacemaker that has an implanted pacing device implanted in a vein or artery of the patient's heart. The pacing device responds to detecting abnormal electrical cardiac activity by applying a voltage pulse across a pair of electrodes, thereby stimulating muscles adjacent the vein or artery which causes contraction of the heart. The implanted pacing device is powered by energy derived from a radio frequency signal received from an extracorporeal power supply. The derived energy charges a capacitor or other storage mechanism in the pacing device which provides electrical voltage for the simulation.
Heretofore, an open loop system was employed to supply power to the implanted device from the extracorporeal power supply. That system was designed to meet the worst case power demand so that the implanted device had sufficient power to function in every situation. However, this resulted in the excessive energy being furnished to the implanted device the vast majority of the time when less power was needed. The open loop system was an inefficient use of the battery power in the extracorporeal power supply.
It is desirable to control the transmission of the radio frequency signal to the implanted medical device in a manner that ensures that its energy storage device always is sufficiently charged without providing excessive energy.